In the field of drilling in the exploration for oil and gas, an important component is that of the formulation of drilling muds. Drilling muds are the fluids which are used to maintain pressure, cool drill bits and lift cuttings from the holes and vary in composition over a wide spectrum. Generally, drilling muds are based on aqueous formulations or oil-based formulations.
A conventional oil-based drilling mud formulation is comprised of basically the following ingredients: oil (generally No. 2 diesel fuel), emulsifying agents (alkaline soaps and fatty acids), wetting agents (dodecylbenzene sulfonate), water, barite or barium sulfate, (weighting agent), asbestos (employed as viscosification agent) and/or amine-treated clays (also a viscosification agent).
The above combination of ingredients is generally formulated to possess various weights based primarily on amount of barite added. For example, a typical drilling mud can vary in specific gravity from a range of about 7 pounds per gallon up to 17 pounds per gallon or even greater. This variation in specific gravity is primarily controlled by the amount of barite added. The above formulations perform adequately in a number of applications, primarily those where the use of oil-based drilling muds is dictated by the lack of stability of the formation in which drilling is taking place. For example, in various types of shale formation, the use of conventional water-based muds can result in a deterioration and collapse of the shale formation. The use of the oil-based formulations circumvents this problem. However, it is observed that the current oil-based drilling muds have some significant disadvantages. One disadvantage is that the incorporation of asbestos or asbestos fines can incur significant health problems, both during the mud formulation and potentially during the subsequent use of such formulations. Therefore, it is desirable to eliminate the use of asbestos completely in such drilling muds. On the other hand, the use of substitutes for asbestos in this application has heretofore not been particularly successful in that the resulting viscosification agents must maintain adequate viscosities under the drilling conditions which can involve high temperature and high shear conditions.
There has been a substantial need for a drilling fluid which would exhibit good performance at high temperature in water sensitive formations. Past experience has shown that oil-based drilling fluids can provide good performance in water sensitive formations, and the state of the art systems can perform well up to about 350.degree. F. Typically, in such formations, the failure of the viscosities in current muds is circumvented by the addition of more viscosifier during the circulation of the drilling mud. While this solution is adequate at moderate temperatures, when much higher temperatures are encountered (example: geothermal wells or natural gas wells), the degradation of the viscosifier can be so rapid that the additional costs for a viscosifier can be uneconomical. There is a need; therefore, for drilling fluids which can maintain their viscosity and gel strength at temperatures up to and exceeding 400.degree. F. These needs are not adequately met by the current drilling fluids, even with the oil-based drilling muds often employed.
This invention describes an approach to viscosification of oil-based drilling muds which permits the substitution of sulfonated ionomers for asbestos fines. The resulting polymer-modified drilling muds display viscosities which are in a desirable range for drilling mud applications, good viscosities at retention, after aging and gel strength, from room temperature up to temperatures of 400.degree. F. and higher, based on tests conducted for 16 hours at such temperatures.
It has also been shown that sulfonated polystyrene is very effective as a viscosifier for oil-based drilling muds at elevated temperatures, as described in copending application Ser. No. 292,333 entitled, "High Temperature Drilling Fluids Based on Sulfonated Thermoplastic Polymers". However, these formulas, as envisioned in Ser. No. 292,333, provide low gel strengths at room temperature to 150.degree. F. and are not effective until thermally activated. We have found that sulfonated EPDM provides good viscosification at temperatures of about 300.degree. F. and below when formulated in a mud based on fresh water. Higher temperatures (350.degree. to 400.degree. F.) encountered in accelerated aging studies for 16 hours have shown that sulfonated EPDM loses its efficacy almost completely at those higher temperatures.
It appears that until now, there has been no viscosifier which is effective at both room temperature and at 400.degree. F. for the typical aging period of 16 hours. The instant invention will describe materials that provide excellent gel strength at both room temperature and at temperatures around 400.degree. F. and at even higher temperatures.
The preferred thermoplastic species of the instant invention is a lightly sulfonated polystyrene with a sulfonate level of about 5 to about 100 meq. of sulfonate groups per 100 grams of sulfonated polymer which is used in combination with an amine-treated clay. It is believed that this polymer is not soluble in either the hydrocarbon phase or the aqueous phase of a formulated mud and; therefore, the polymer is probably located at the interface. As a consequence, this polymer is an extremely effective viscosifier even at quite low levels.
A second facet of the instant invention relates to the use of these materials in formations which employ high concentrations of salt in the aqueous phase. The mixture of amine-treated clay and sulfonated polystyrenes (SPS), which are the preferred embodiments of this invention, lose some of their efficacy in salt water. It has been found that the combination of a suitable non-ionic emulsifier with the sulfonated polymer and amine-treated clay gives formulations which are effective with salt water. Therefore, these systems give formulations which perform well at high temperatures and in the presence of salt water phases, which is a highly desired objective in the drilling fluids industry.